Looking For Help With Counter Melodies...

I have recently started trying to make orchestral music, on my laptop.

And I was wondering if anyone could help me to better understand, how to make

successful (and decent) sounding counter melodies.

My attempts so far have not been very successful...so if anyone has any advice

for me, it's greatly appreciated.

get a music sheet or a midi of mozart and see how he did it

Are you just going by ear right now? Because the process of trial and error to find what sounds good will be very tricky and time-consuming. I think it would be easier to learn the principles of counterpoint. In general, it's probably good to have the melody and counter melody moving in contrary or oblique motion so that they are independent from each other. 

Daniel, I don't know where you are in your musical studies, and although I could offer  you "ideas," etc, I won't. If you're serious about music you need to find a teacher, not a blog or a website or even a book. You need to find a teacher. That's the best advice I can give to you.